1. Field of the Invention
The present invention relates to a method of anaerobic treatment and anaerobic treatment apparatus which treat water containing organic matter by using methane fermentation, and in particular, the present invention is related to a method of anaerobic treatment and apparatus in which water containing organic matter is introduced to a reaction tank which retains granular sludge and an anaerobic biotreatment is performed.
2. Description of the Related Art
A UASB (Upflow Anaerobic Sludge Blanket) process which performs a high-load, high-speed treatment using a high density granular sludge having excellent settling properties is known as an anaerobic treatment method of an organic waste water. In the UASB process, waste water is introduced to a reaction tank which retains a sludge blanket formed by the granular sludge and by passing a liquid in the upflow, contacts with the sludge blanket. In this method, in order to perform a high-load high-speed treatment, organic solids with a slow digestion rate are separated in the case where they are contaminated in the wastewater, and dissolved organic matter with a high digestion rate is biologically treated by the granular sludge. As a treatment method which develops the UASB method and makes a more high-load high-speed treatment possible, an EGSB method (Expanded Granule Sludge Blanket) is also known in which wastewater r is passed at a higher flow rate into a tall reaction tank and the sludge blanket is developed at a high development rate.
In the UASB and EGSB methods, a granular sludge in which anaerobic microorganisms are granulated is used and the sludge containing anaerobic microorganisms is maintained in a granular form and multiplied. A biological treatment method which uses granular sludge can be operated at high-load high-speed because a high sludge retention concentration can be obtained compared to a fixed bed or fluidized bed which retain the microorganisms on a support material. In addition, because the density of microorganisms is high and settling properties are excellent in granular sludge, solid liquid separation is easy. Furthermore, it is recognized as the most efficient anaerobic treatment method because excess granular sludge from an operating reaction tank can be transferred as seed sludge for a new reaction tank, the reaction tank can be started in a short time and a stable treatment can be performed from the start-up period.
In a method such as UASB which uses granular sludge, the most important point for treating stably and successfully wastewater is maintaining and multiplying the granular sludge. If the granular sludge can not be maintained or multiplied within the reaction tank, the ability to treat the wastewater is reduced and eventually becomes impossible to treat.
Granular sludge is formed when microorganisms of an aceticlastic genus Methanosaeta (formerly called genus Methanothrix) become a skeleton, forms one type of ecosystem in which hydrogenotrophic methanogens, acetic acid bacteria, and acidogens coexist. Even within these microorganisms, acidogens work towards increasing the binding strength between bacteria because they catabolize glucide, lipids and proteins, and produce a viscous substrate. Therefore, if the culture of a sugar substrate is used the strongest granular sludge is produced.
Because wastewater r in general sewage or industrial effluent contains glucide and other high molecular organic matter, acidogens multiply when this is anaerobically treated. In the anaerobic treatment process, the above stated microorganisms other than the acidogens also multiply, organic acids are produced and the organic acids become acetic acid by being catabolized in sequence and further catabolized into methane and carbon dioxide gas. Under the condition that the acidogens multiply, each type of microorganism stated above which participates in the series of anaerobic treatments stated above and multiplies, binds together via the viscous substrate and a very strong granular sludge is formed. Therefore, by passing general wastewater in the upflow and performing anaerobic treatment it is possible to form self generating granular sludge.
However, unlike this usual wastewater r, when a liquid to be treated which has a low substrate concentration of acidogens, for example, a liquid to be treated containing organic matter below carbon number of four which are discharged from a chemical factory etc., is treated, the granular sludge becomes easier to disintegrate. In particular, in the case where a liquid to be treated in which the main components (about 50-90% by mass of total organic matter) are organic matter below carbon number of two such as acetic acid, methanol, ethanol and acetaldehyde, microorganisms multiply with genus Methanosarcina as the dominant species.
It is difficult for bacteria of genus Methanosaeta, genus Methanosarcina and genus Methanobacterium to form granular sludge, and because the production of a viscous substrate in the granular sludge becomes less, multiplication of the granular sludge becomes anemic and strength is also insufficient. As a result, when a long period of operation is continued of water containing these types of organic matter as the liquid to be treated, the granular sludge is broken down, has a small particle size and the amount of sludge within the reaction tank is reduced.
In particular, in the case of treating a liquid to be treated which has a substrate having carbon number of one, specifically methanol, formic acid, or formaldehyde as the main component, because the above stated genus Methanosaeta can not utilize these substrates, Methanogens of genus Methanosarcina or genus Methanobacterium are grown and become a state which is more difficult to granulate. For example, when methanol as a single substrate is used in a USAB type treatment apparatus which is continuously operated for a long period of time, the granular sludge breaks down, is miniaturized and the amount of sludge is dramatically reduced. As a result, anaerobic treatment with granular sludge of a liquid to be treated which has low molecular organic matter such as those stated above as the main component was difficult.
A treatment method which supplies acetic acid or a substance which produces acetic acid when an anaerobic treatment using a granular sludge such as USAB is initiated, has been proposed (for example, Japanese Registered Patent No. 2563004). In the method disclosed in Japanese Registered Patent No. 2563004, when an apparatus for treating wastewater which are difficult to granule as stated above, is initiated, acetic acid or a substrate which produces an acetic acid is provided, methanogens of genus Methanosaeta are caused to dominatedly multiply and the granular sludge is caused to disintegrate in a short period of time.
However, because genus Methanosaeta can use a low molecular compound as a substrate, if the introduction of acetic acid is terminated after initiating an anaerobic treatment using granular sludge using the method disclosed in patent document 1, methanogens of genus Methanosarcina or genus Methanobacterium gradually grows, and break down of the granule begins.